1. Field of the Invention
This invention relates to a pneumatic radial tire having excellent cornering properties and either or both of durability and high-speed durability. More it relates to a pneumatic radial tire for use in general-purpose passenger cars mainly developed for weight reduction.
2. Description of the Related Art
With the demand for energy saving, it has been attempted to improve the fuel consumption in automobiles by reducing the weight, and this in turn tends to raise the demand for weight reduction in tires. Particularly, this is true in pneumatic radial tires for use in general-purpose passenger cars.
In general, a pneumatic radial tire comprises a so-called cross belt formed by laminating at least two slant belt layers on an outer periphery of a carcass crown portion to cross cords of these layers with each other. From a viewpoint of tire weight reduction, there have been developed tires comprising a belt composed of a single slant belt layer and a circumferential belt layer(s) containing light-weight organic fiber cords arranged substantially in parallel to an equatorial plane of the tire. The tire of this type is disclosed in, for example, JP-A-62-152904 and JP-A-4-163212.
In the tires disclosed in these prior art publications, organic fiber cords having a high tensile rigidity in the circumferential direction such as aromatic polyamide fiber cords (e.g. Kevlar cord) and the like are used as a cord in the circumferential belt layer. Further, it is disclosed that the circumferential belt layer can control the pushing out of the tread portion through centrifugal force during the high-speed running to improve the high-speed durability.
Moreover, organic fiber cords widely used as a cord for a carcass ply such as polyethylene terephthalate (hereinafter abbreviated as PET) fiber cord, nylon fiber cord, polyethylene naphthalate (hereinafter abbreviated as PEN) fiber cord and vinylon (PVA) fiber cord are considerably low in tensile rigidity in the circumferential direction as compared with the aromatic polyamide fiber cord, so that they are hardly even applied as a cord for the circumferential belt layer. Also, the use of steel cords for the circumferential belt layer is unfavorable from a viewpoint of weight reduction and the like, so that steel cords are hardly even used in the circumferential belt layer.
When aromatic polyamide fiber cords are used in the circumferential belt layer, however, the resistance to compression fatigue is poor, so that it has been confirmed that the belt is apt to create compression breakage accompanied with buckling deformation of the belt generated during the severe cornering.
The inventors have made various studies with respect to the use of organic fiber cords in the circumferential belt layer and found that PET, nylon, PEN and vinylon fiber cords, which have never been used in the circumferential belt layer in the past, can be used in the circumferential belt layer by rationalizing the twisting structure, total denier and twist constant of such a cord, and also high-strength steel cords formed by rationalization of the twisting structure can be used in the circumferential belt layer without increasing the tire weight so much.
It is, therefore, an object of the invention to provide pneumatic radial tires having excellent cornering property and either or both of durability and high-speed durability by using rationalized PET fiber cords, nylon fiber cords, PEN fiber cords, vinylon fiber cords or steel cords in the circumferential belt layer, particularly pneumatic radial tires for use in general-purpose passenger cars for attaining the reduction of tire weight.
According to the invention, there is provided of a pneumatic radial tire comprising a carcass toroidally extending between at least a pair of bead cores and a belt superimposed on a crown portion of the carcass and comprised of one slant belt layer containing a plurality of cords or filaments arranged at a given inclination angle with respect to an equatorial plane of the tire and at least one circumferential belt layer laminated on the slant belt layer and containing a plurality of cords arranged substantially in parallel to the equatorial plane, an improvement wherein the cord used in the circumferential belt layer is made from PET, nylon, PEN or vinylon fiber and has a layer twisting structure and a total denier DT of 1000 d-6000 d and a twist constant Nt represented by Nt=Txc3x97(0.139xc3x97DT2xc3x971/xcfx81)xc2xdxc3x9710xe2x88x923 of not more than 0.3 in case of PET or nylon fiber, not less than 0.5 in case of PEN fiber, or not less than 0.6 in case of vinylon fiber, wherein T is a twisting number (twist turns/10 cm) and xcfx81 is a specific gravity.
The term xe2x80x9clayer twisting structurexe2x80x9d used herein means that one or more fibers are twisted in a certain direction (cable twist) and two or more fibers are twisted in a direction opposite thereto (ply twist).
The term xe2x80x9ctotal denierxe2x80x9d used herein means a product of fiber denier and number of fibers to be twisted.
It is preferable that the cord for the circumferential belt layer has a loss tangent (tan xcex4) of not more than 0.3 under conditions that an initial tension is 1 kgf/cord, a strain amplitude is 0.1%, a frequency is 20 Hz and an atmosphere temperature is 25xc2x0 C.
In case of requiring greater circumferential rigidity in the circumferential belt layer as compared with the case of using the PET, nylon, PEN or vinylon fiber cords, the cord in the circumferential belt layer is preferably a steel cord having a modulus of elasticity of not less than 3000 kgf/mm2 and a twisting structure of 1xc3x97N or 1+N in which N is the number of steel filaments.
In preferred embodiments of the invention, a coating rubber for the circumferential belt layer has a modulus of elasticity of not less than 200 kgf/mm2, the cord for the circumferential belt layer is spirally wound around the slant belt layer, and the slant belt layer contains steel cords or steel filaments therein. Further the slant belt layer has a cord inclination angle of 15-45xc2x0 with respect to the equatorial plane, and a gauge of rubber located between the cord of the slant belt layer and the cord of an innermost circumferential belt layer in a radial direction of the tire is made larger in a side end region of the tire than in a central region thereof in the widthwise direction at a radial section of the tire. Further a gauge of rubber located between the cord of an outermost circumferential belt layer in the radial direction and an inner periphery of a tread rubber is made larger in the central region of the tire than in the side end region thereof, at least two circumferential belt layers are disposed in the central region in the widthwise direction of the tire, and at least two circumferential belt layers are disposed in the side end region in the widthwise direction of the tire.
Moreover, the modulus of elasticity of the coating rubber for the circumferential belt layer is calculated from a relationship between load and displacement. This is accomplished by compactly filling a rubber specimen 9 in a cylindrical cavity of a steel jig 8 having a diameter d of 14 mm and a height h of 28 mm as shown in FIG. 7a, setting the jig 8 into a compression testing machine 10 as shown in FIG. 7b and applying a load W onto upper and bottom faces of the rubber specimen 9 at a rate of 0.6 mm/min to measure a displacement by means of a laser displacement meter 11.